1. Field of the Invention
The present invention relates to an image forming apparatus which transfers a toner image on an image bearing member onto an intermediate transfer member, and transfers the toner image on this intermediate transfer member onto a transfer material.
2. Description of the Related Art
Various electrophotographic image forming apparatuses have conventionally been proposed. FIG. 1 illustrates an entire configuration of a conventional electrophotographic image forming apparatus. This image forming apparatus G1 has a photosensitive drum 1 which is rotatably supported and rotation-driven in the arrow direction. On the outer periphery of the photosensitive drum 1, there are arranged a primary charger 2 which charges uniformly the surface of the drum, exposing means 3 such as a laser beam exposure unit which forms an electrostatic latent image on the drum surface by irradiating a color-separated optical image or one corresponding thereto, a developing device 5 which converts the forgoing electrostatic latent image into a visible image (toner image), an intermediate transfer member 6 onto which the toner image is transferred under the pressure of the photosensitive drum, and a cleaner 7 which removes toner remaining on the drum surface.
The intermediate transfer member 6 has a layer structure as shown in FIG. 4B: a conductive layer 12 and a surface layer 13 onto which the toner image is transferred are provided on a conductive substrate 11. When transferring the toner image present on the photosensitive drum 1 onto the intermediate transfer member 6, a prescribed bias is applied by a power supply 21 onto the substrate 11.
A convey belt 9 for conveying a sheet material P so as to be in contact with the intermediate transfer member 6 is arranged below the intermediate transfer member 6, and a fig device 8 for performing fixing of the toner image is arranged downstream of the convey belt 9.
Now, operations of the aforesaid image forming apparatus will be briefly described below.
When the primary charger 2 is operated in a state in which the photosensitive drum 1 is rotation-driven, the surface of the drum is uniformly charged. When the exposing means 3 irradiates an optical image, an electrostatic latent image is formed on the drum surface, and this electrostatic latent image is converted into a visible image by the developing device 5. Then, the visible image Is transferred onto the intermediate transfer member 6.
In the meantime, the sheet material P is transferred by the convey belt 9, and the toner image on the intermediate transfer member 6 is transferred onto the sheet material P by a transfer roller 20. Thereafter, the sheet material P is sent to the fixing device 8 for fixing of the toner image, and is discharged to outside the apparatus.
The toner remaining on the surface of the photosensitive drum 1 is, on the other hand, removed by the cleaner 7, and the photosensitive drum 1 is now ready for the next image forming process.
The photosensitive drum 1 has a conductive substrate made of a metal or the like and a photosensitive layer formed on the surface of the substrate. An intermediate layer such as an adhering layer or a conductive Layer may be provided between the conductive substrate and the photosensitive layer. These layers including an intermediate layer, if any, are hereinafter referred to as the photosensitive layer The photosensitive layer of the photosensitive drum 1 has a longitudinal length substantially equal to the longitudinal length of the intermediate transfer member 6.
In the foregoing image forming apparatus, a problem is encountered in that discharge (electric stress) occurs at the both ends of the region where the photosensitive drum 1 comes into contact with the intermediate transfer member 6, thus leading to image blurs in a transfer image of the intermediate transfer member 6 or that on the sheet material P.
An organic resin is commonly used for the photosensitive layer for various reasons (cost reduction, wide selection of materials, for example). The aforesaid problem is more apparent since an organic resin has poor mechanical strength and is easily susceptible of resin deterioration caused by discharges.